Reactive Etching of Positive and Negative Silicon Cluster Ions By Nitrogen Dioxide.

Positively and negatively charged silicon cluster ions react exothermically with NO2. The predominant reaction is loss of a single silicon atom in the form of SiO. This reaction repeats sequentially such that the clusters are "etched" down to monatomic silicon ions. This is the first case where the same reactivity has been found for both the positive and negative ionic forms of a sequence of clusters. The ionic silicon cluster reactions with NO2 are explained by a radical-radical coupling mechanism involving the unpaired electron on NO2 and an unpaired electron on a silicon atom at a charged or trivalent center in the clusters. Involvement of a radical electron on the silicon cluster contrasts with ionic silicon cluster reactivity with CH3SiH3 which requires divalent reactive centers on the clusters. Previously identified "magic number" clusters do not exhibit a typical reactivity.